Conventional cable modem systems use any of the conventional 6 MHz bandwidth television channels for downstream communications, i.e., from the cable modem terminating system to the cable modems. Such television channels are at relatively high frequencies, e.g., 60 MHz and higher. For the upstream communication, i.e., from the cable modems to the cable modem terminating system, relatively low frequencies, e.g., 5–42 MHz, are employed.
The reason that use of such low frequencies has become ubiquitous for upstream communication is because cable was originally only a downstream medium and historically it was deemed desirable to maximize the number of available downstream channels for television use. As the amplifiers that were used in the cable networks were bandwidth limited, and so only could only provide so many channels from the lowest frequency channel allowed by FCC rules in the channel plan to the highest frequency channel that could be amplified by the bandwidth limited amplifier, all the available channels were assigned for downstream use. When it became recognized that upstream communication was desirable, the only spectrum that was available, without sacrificing available television channels, was the bandwidth below the lowest channel in the channel plan. As a result, notwithstanding that higher bandwidth amplifiers that are available today, the art continues to essentially use for the upstream communication only the low frequencies that are below the lowest channel in the channel plan.
Unfortunately, such low frequencies are often subject to noise, which increases the bit error rate and reduces throughput. This noise is typically only within certain frequency ranges. So long as the frequencies selected for the uplink is not within the range of the noise, upstream communication can take place relatively error free.
In an attempt to insure that the upstream communication takes place using an upstream channel that is not in the frequency range of the noise, the prior art teaches that it is possible to change the channel or “hop” from one channel to another. According to one prior art technique the hopping is performed according to a predefined schedule which does not take into account actual conditions on the cable. According to another prior art technique, when the bit error rate is detected to have increased, transmission is halted and a “snapshot” of the channel conditions is taken using a digital signal processor (DSP) which is included in each cable modem terminating system module, also known as a port adapter, which is the head end cable modem that serves a predefined geographic area. The “snapshot” of the channel conditions looks across the whole upstream frequency range, e.g., 5–42 MHz, and determines the power present at each frequency. Using the snapshot it selects the widest possible channel with the lowest power and over the downstream instructs all the cable modems it serves to hop so as to use that channel for the upstream, to which it also tunes.